A few things dirges provide DR and PR, while troubs do AR. Dirges give the group a disease proc, troubs a mental proc.
Dirges life tap, troubs mana tap.
Dirges a rez, troubs a charm, not a good one though.
Dirges get more debuffs, while troubs get more buffs.
Both have their ups and downs, I like my dirge. The life taps help soloing though they have some stupid abilities. Why would a scout need a taunt that requires concentration, or a spell to make your target run for a few seconds. Since you can be either look at the spell lists
and see what you prefer.